1. Field of the Invention
Embodiments of the present invention relates to a system and method for generating narrowband Lamb waves for use in, for example and not limitation, non-destructive testing. Specifically, embodiments of the present invention relate to the non-contact generation of Lamb waves in thin plates using laser beams and software analysis to analyze weld parameters including, but not limited to, weld penetration.
2. Background of Related Art
Butt joint welding is an essential process of joining parts in many industries. The schematic of cross section of a butt weld is shown in FIG. 1, which depicts a variety of weld dimensions including penetration depth (PD), reinforcement height (RH) and bead width (BW). Among them, PD is an important geometric parameter that indicates weld quality and is used as a key quality control quantity. The evaluation of PD in butt welds in thin plates, therefore, has many practical applications. Conventionally, cutcheck, i.e., physically cutting the sample across the weld, has been widely used to monitor weld quality. This procedure, however, is time-consuming, destructive, and wasteful. In addition, automated inspection using cutcheck is not possible.
For at least the preceding reasons, it is desirable to perform non-destructive testing (“NDT”) on a variety of materials to detect and locate, for example and not limitation, material defects, manufacturing defects, and weld quality. As a result, considerable resources have been invested to develop NDT methods such as, among other things, ultrasonic inspection, radiography, thermography, and eddy current inspection.
Ultrasonic inspection techniques have gained greater acceptance for a variety of purposes in recent years. It is one of the major techniques used, for example, for inspection of welds in structures. Conventionally, contact piezoelectric transducers (PZTs) have been used to generate and receive ultrasounds during offline, as opposed to real-time, sample inspection. Due to the need for liquid couplants between the PZTs and the sample, however, this method is not suitable for automated real-time inspection during manufacture.
Non-contact ultrasonic sensing, on the other hand, has the potential to detect defects and discontinuities in real time. Using laser generated ultrasounds and an electromagnetic acoustic transducer (EMAT) receiver, for example, is one method suitable for both offline and real-time sample quality monitoring. Nanosecond pulse width lasers such as, for example, Q-switched Nd:YAG lasers can be used to generate ultrasound.
In use, a high energy, very short duration pulse from the laser induces a rapid increase in the local temperature of the sample. The heated region expands thermoelastically and then slowly contracts when the laser pulse is momentarily shut off. The rapid expansion and slower contraction creates ultrasounds which propagate through the sample. In addition to the thermoelastic effect, ablation can occur if the energy of the laser pulse is increased to the point that some portion of the surface evaporates. The ultrasounds generated in the ablation regime are much stronger than those generated in the thermoelastic regime, though the latter is generally preferred for true NDT.
Conventionally, a laser or a laser phased array system has been used to generate ultrasounds (i.e., bulk waves) to measure various characteristics in thick structures (e.g., weld penetration). A Time of flight diffraction (TOFD) technique can be used to evaluate, for example, material defects or weld characteristics. By measuring the arrival time of an ultrasonic signal, for example, various characteristics of weld such a penetration depth can be measured.
When the thickness of the sample approaches the wavelength of the ultrasonic wave, however, this method no longer provides accurate data. For thin materials, ultrasonic waves give way to Lamb waves, which exhibit very different characteristics compared to the bulk waves that travel in thick structures. Lamb waves travel through the cross section of the structure, are dispersive, and their traveling speeds are dependent on their frequencies. Lamb waves are widely used in structural integrity inspection and defect detection in thin structures because of their potentials to inspect large area and their sensitivity to a variety of damage types.
The use of lasers to generate Lamb waves is beneficial due to its noncontact nature. Laser generated ultrasound is broadband in nature, however, and this, combined with the dispersive nature of Lamb waves, makes signal processing complicated. To simplify signal processing in thin structures, therefore, narrowband Lamb waves are desirable.
Conventionally, this has been achieved using spatial array illumination sources produced by, for example, shadow masks, optical diffraction gratings, multiple lasers, interference patterns, and lenticular arrays. Shadow masks, depicted in FIG. 3a, are economical, fairly effective and easy to implement (hereinafter referred to as “pattern source”), but they are not flexible and have several disadvantages. These include, but are not limited to, the need to fabricate different masks for each different wavelength of interest, the absorption of a substantial amount of energy by the mask, and the inability to practically manufacture masks with very small spacing. In addition, because the masks must be manually changed for each separate wavelength, experimental setup for masks for a large number of wavelengths can be impractical.
With respect to the analysis of welds in particular, the relationship between the reflection coefficients of Lamb wave modes and geometry of notches with varying width or depth in thin plates has been investigated. Some previous methods include the boundary element method and the finite element method to study reflection coefficients of fundamental A0 and S0 Lamb wave modes from a notch. Previous investigation has shown that reflection coefficients of Lamb waves are not only dependent on the geometry of the notches, but also on the wavelengths of Lamb waves. The Study of guided waves traveling in elastic plates with Gaussian section variation showed that waves can be trapped in the Gaussian domain depending on the incident mode and on the Gaussian maximum height.
The geometry of a butt weld can be approximated as a plate with Gaussian section variation and a notch. No analytical solutions or models can be found in the literature, however, to describe how Lamb waves propagate in this kind of structure. The problem is further complicated by the existence of the material interfaces between the weld bead and the base material. Prior to the development of embodiments of the present invention, all that was known is that the reflected waves contain information regarding weld dimensions. Utilizing this information remained a mystery.
What is needed, therefore, is a system and method (“system”) for efficiently creating narrowband Lamb waves using a focused energy source (e.g., one or more laser sources). The system should retain the non-contact benefits of conventional pattern source methods, but provide improved flexibility and efficiency. The system should reduce wave complexity using various mathematical methods to enable analysis of wave behavior for NDT of, for example, butt welds in thin plates. It is to such a system and method that embodiments of the present invention are primarily directed.